The present invention relates to a new and improved construction of stepwise control or regulator for on-off or two-step control incorporating switching stages, which for each stage value of a control magnitude as the input magnitude of the corresponding stage value switches-in a physical output magnitude as an adjustment magnitude and each of which possesses switching hysteresis, the width of which is determined by the difference of the magnitude of the input values for switching-in and switching-out the switching stages. Further, there is present a smallest stage value of the output magnitude and the output magnitude value of a subsequent stage always is greater by this smallest stage value than the preceding stage and the total value of the output magnitude at the switching stages is subdivided into such partial values that through switching-in and/or switching-out the switching stages there can be portrayed the individual stage values of the output magnitude.
As is well known in a two-point or on-off control, the reference value continuously alternately exceeds or drops below the regulating or control magnitude, so that the course of the control produces a continuous oscillation of the magnitude to be controlled. For the control equipment and the control or regulation function there are of significance in this regard the oscillation period and the amplitude of the control oscillations. The oscillation period determines the switching frequency during the control and is therefore essential for the longevity of the switches containing, for instance, protective or overload relays. The amplitude is represented by the difference between the upper and lower boundary values of the magnitude to be controlled and which occur during the control operation and indicate the amount by which the reference value has been exceeded or fallen below. In the case of an on-off control at a control system with equalization and recovery or dead time, the amplitude of the control oscillation is dependent upon the hysteresis width and by virtue of the parameters of the control system (recovery time, time-constant, gain) upon the speed of change of the control magnitude. The greater the width of the hysteresis and the speed of change of the control magnitude, that is to say, in the case of a control system of predetermined recovery time of the magnitude of the switched-in and switched-out value of the output magnitude, that much greater is also the amplitude of the control oscillations. In order to be able to obtain small amplitudes of the control oscillations, there is therefore permitted to be continuously switched-in a certain proportion of the output magnitude as the base load and there is always switched-in and switched-out, for control purposes, only a small portion thereof. For such control operations there have been developed stepwise controls or regulators.
With the known decimal stepwise controls, the total value of the output magnitude is uniformly divided over a number of switching stages, so that each one cuts-in and cuts-off the same stage value of the output magnitude. If with a constant disturbance magnitude there is required a value of the output magnitude which is exactly equal to the number of stage values in order to maintain a condition of the control system at the stepwise control, which condition is determined by the reference value of the control magnitude, then by means of the input magnitude there is switched-in through the agency of a network, just so many switching stages in succession until there is attained the required value of the output magnitude. After this start-up phase, there does not occur any further control. If for a constant disturbance value there is required, for maintaining the control system condition, a value of the output magnitude which is between two stage values, in other words is composed of a number of stage values and a fraction of a stage value, then, owing to the control magnitude in the start-up phase, there are switched-in a corresponding number of switching stages and the still missing fraction is maintained, in that the next successive switching stage is periodically switched-in and switched-out, wherein during the oscillation period of the control oscillation which adjusts itself this switching stage is respectively once switched-in and switched-out and there is a behavior of the switch-in time of the switching stage to the oscillation period like the fraction to the stage value of the output magnitude. Small changes in the disturbance magnitude only alter the switch-in time of the switching stage within the oscillation period. Larger changes in the disturbance magnitude produce a behavior of the control like during the start-up phase, that is to say, the switching stages are switched-in and the switched-out respectively. In consideration of a P-regulator (proportional control or regulator) it is therefore possible to define for the stepwise control a proportionality region, and specifically a static proportionality region or a static proportionality band, the width of which is defined by the amount of the deviation of the control which is necessary in order to place all of the switching stages of the control out of the switched-out state into the switched-in state, and a dynamic proportionality region or a dynamic proportionality band, the width of which corresponds to the value of the control deviation which is determined by the cut-on point (or cut-off point) for two successive switching stages. The proportionality regions, just as the amplitude of the control oscillations, can be defined in terms of units of the input magnitude of the control and it is apparent that the dynamic proportionality region can not be greater than the amplitude of the control oscillation, otherwise during the control more than one switching stage will be switched and the control will become instable. The width of the dynamic proportionality region is also dependent upon the hysteresis width of the associated switching stages. Due to the tolerances of the components of the switching stages, their hysteresis width is practically never equal, so that with the decimal stepwise control there are also present different widths of the dynamic proportionality regions, which is undesirable for certain controls.
The smaller the control width for a given adjustment region, that is to say, the smaller the width of the dynamic proportionality region for a given static proportionality region, the more switching stages are required. It can happen that for control functions with a decimal stepwise control, there are necessary for instance thirty and more switching stages. This is uneconomical and a considerable drawback of the decimal stepwise control, particularly if it is considered the expenditure of hardware at the input network also increases as a function of the number of switching stages. Since with the decimal stage control the total value of the output magnitude is uniformly subdivided over its switching stages and during the control operation always only one switching stage and therefore only a corresponding small partial amount of the output magnitude is switched-in and switched-out, the large number of switching stages of such control also can be of advantage, for instance when the output magnitude is a high electrical output. Thus, for the example under consideration with an output magnitude of, for instance, 30 kW during the control via the one switching stage always only 1 kW is switched-in and switched-out and thus the network is only slightly loaded.
The number of required switching stages can be reduced in that the total value of the output magnitude can be divided according to a binary code over the individual stages. Such distribution of the output magnitude occurs with the known binary stepwise controls. In accordance with the desired width relationship of the dynamic to the static proportionality region or the desired relationship of the control width to the adjustment width, in the case of the binary stepwise control, the total value of the output magnitude is subdivided into 2.sup.n -1 equal stage values, wherein n represents the number of required switching stages. With each switching stage there is associated a partial value of the output magnitude, which is equal to 2.sup.n.sup.-1 P, wherein n = 1,2,3 . . . , the stage number, and P is the stage value of the output magnitude. With a given relationship of the control width to the adjustment width of approximately for instance 1:30 there is thus required for a binary stepwise control 5 switching stages (2.sup.5 -1 = 31). The stage value P of the output magnitude amounts to 1/31 of its total value and the individual switching stages have associated therewith the output magnitude-partial values 1P, 2P, 4P, 8P and 16P, that is to say, 1/31, 2/31, 4/31, 8/31 and 16/31 of the total value of the output magnitude. The switching stages are switched by the "decimal" prevailing input magnitudes via an input network which, for each stage value of the input magnitude, switches-in those switching stages whose partial values of the output magnitudes collectively produce the corresponding stage value. Since for a given stage value the input magnitude of a number of switching stages must be generally simultaneously switched (for instance for the input stage 7 the switching stages 1/31, 2/31, and 4/31) the construction of the input network is more complicated than in the case of the decimal stepwise control and the switching frequency for the individual switching stages is greater. The switching stages themselves (motor or electromagnetic stage switches) of a binary stepwise control do not differ from that of a decimal stepwise control and also the course of the control for a control system with a binary stepwise control essentially corresponds to the control course of one with decimal stepwise control. If for maintaining a control system condition, there is required for instance as much as one-half of the value of the output magnitude, then for instance with a 30-stage decimal stepwise control the first 15-switching stages are switched-in, with a 5-stage binary stepwise control, on the other hand, there are switched-in the first four switching stages, which collectively amount to 15/31, that is to say not quite one-half of the value of the output magnitude, and the fifth switching stage, at which with 16/31 there is dispensed with more than one half of the output magnitude, is alternately switched-in and switched-out via the input network. With the binary stepwise control, it is no longer possible to state that during the control a base load is continuously maintained switched-in, rather in the most unfavorable situations about 50 percent of the value of the output magnitude is continuously switched in and switched-out. This is particularly then disadvantageous if, for instance, the output magnitude, as mentioned, constitutes a high electrical output, since the network during the control operation is strongly loaded. Furthermore, with the binary stepwise control, the dynamic proportionality region does not possess a uniform width, which likewise can be disadvantageous.
Generally, it is therefore necessary for a given control problem to initially decide whether advantageously there should be employed a decimal or a binary stepwise control.